Boron carbide based materials and process for the fabrication thereof

ABSTRACT

Disclosed is a method for fabricating a solid article from a boron carbide powder comprising boron carbide particles that are coated with a titanium compound. Further disclosed herein are the unique advantages of the combined use of titanium and graphite additives in the form of water soluble species to improve intimacy of mixing in the green state. The carbon facilitates sintering, whose concentration is then attenuated in the process of forming very hard, finely dispersed TiB 2  phases. The further recognition of the merits of a narrow particle size distribution B 4 C powder and the use of sintering soak temperatures at the threshold of close porosity which achieve post-HIPed microstructures with average grain sizes approaching the original median particle size. The combination of interdependent factors has led to B 4 C-based articles of higher hardness than previously reported.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional under 37 C.F.R. § 1.53(b) ofprior U.S. patent application Ser. No. 13/976,724, filed Aug. 27, 2013,which in turn is a 35 U.S.C. § 371 National Phase conversion ofPCT/US2011/067618, filed Dec. 28, 2011, which claims priority of U.S.Provisional Patent Application No. 61/427,707, filed Dec. 28, 2010. Thecontents of each of these applications are incorporated in full byreference herein.

FIELD OF INVENTION

The present invention relates to a boron carbon-based material and aprocess for the synthesis of the material.

Definition

Weight percent or wt % as used herein is referring to the relativeproportion of a constituent in a mixture of only the constituent andboron carbide (not the weight percentage of the constituent in a masscontaining boron carbide, the constituent and other constituents). Thus,wt % Ti or titanium, for example, is referring to the percentage of thetitanium in titanium and the boron carbide mass.

BACKGROUND

Boron carbide (B₄C) is the third hardest known material behind diamondand the hardest known material which is thermodynamically stable atambient pressures. High hardness combined with low weight (theoreticaldensity: 2.52 g/cm³) have made boron carbide a preferred material of thestrike face for personal armor systems to stop, for example, armorpiercing bullets. Such armor is typically manufactured by hot-pressing,which involves uniaxially pressing boron carbide powder in graphiteperforms with graphite dies at elevated temperatures. While this processproduces densified ceramic bodies of adequately low porosity to properlyfunction as armor, hot pressed boron carbide is only economicallyfeasible for the fabrication of simple shapes, which can nest one abovethe other so that a stack of parts can be simultaneously hot-pressed.

In a recently issued patent, Speyer et al. (U.S. Pat. No. 7,517,491)showed that high purity sinter-grade (median particle size, d₅₀=0.8 μm)boron carbide could be pressurelessly sintered to a closed porositystate in part by extracting the boron oxide layer on particles in thepressed compact. Such articles could then benefit from the densificationaction of hot isostatic pressing (HIPing). These articles reached neartheoretical density after HIPing, suffered no shape restriction becauseof the method of processing, and yielded hardnesses higher than anyother reported for boron carbide. The boron carbide powder used bySpeyer et al. was high purity, sub micron (d₅₀=0.8 μm), with a broadparticle size distribution which facilitated high green relativedensities (65-70%).

Pressureless sintering a boron carbide compact that includes a sinteringpromoter such as carbon is well known. It is also well known that thecompact can include titanium compounds such as titania which isintroduced into the compact in powder form prior to forming the compact.See C. S. Wiley, Ph. D. Thesis, Synergistic Methods for the Productionof High Strength and Low Cost Boron Carbide, Georgia Institute ofTechnology, Published December 2011.

BRIEF DESCRIPTION OF THE INVENTION

A process according to the present invention can be employed for thefabrication of high hardness solid articles that are suitable, forexample, for the strike face of an armor. A process according to thepresent invention uses a boron carbide powder comprising boron carbideparticles that are coated with a titanium compound, which are formedinto green bodies, pressureless sintered and HIPed to obtain highrelative density and high hardness solid articles that include at leasta boron carbide phase and a titanium diboride phase. Furthermore, acarbon containing compound can be added to the coatings according to thepresent invention to obtain solid articles with unique and advantageousproperties. As discussed in great detail below, the addition of carbonfacilitates sintering, while the formation of titanium diborideattenuates the concentration of carbon to obtain a microstructure thatincludes very hard, finely dispersed TiB₂ phases. Furthermore, it isshown herein that using a narrow particle size distribution B₄C powderand the use of pressureless sintering soak temperatures at the thresholdof closed porosity achieves post-HIPed microstructures with averagegrain sizes approaching the original median particle size of thestarting boron carbide powder.

A process according to the present invention includes a step forprocessing a boron carbide based powder, forming a green body from thepowder, applying thermolysis and pyrolysis steps to the green body,pressureless sintering of the green body to obtain a body sintered to aclosed-porosity state, cooling the pressureless sintered body, and hotisostatic pressing (HIPing) of the cooled pressureless-sintered body.

According to one aspect of the present invention, powder processingincludes forming a preferably aqueous slurry from a combination of aboron carbide powder, a water soluble organometallic source for titanium(e.g. dihydroxybis (ammonium lactate) titanium (IV), C₆H₁₀O₈Ti.2(NH₄)),and, in some embodiments, a water soluble carbon source (e.g. a watersoluble phenolic resin). The slurry is then spray dried to obtain aboron carbide based granulated powder composition, comprised of boroncarbide particles that are coated with a titanium compound and, inembodiments that use a carbon source, a carbon containing compound.

According to another aspect of the present invention, the boron carbidepowder can have a narrow particle size distribution. In some cases, thenarrow particle size may allow for reduction in the cost of startingmaterials. In addition, narrow particle size distribution can retardgrain growth. In the preferred embodiments, the median particles size ofthe boron carbide powder may be less than 1 μm.

According to yet another aspect of the present invention, the boroncarbide-based powder is mixed with a suitable water-soluble binder aspart of the spray drying slurry. After spray drying, uniaxial andisostatic pressing, the green body is then heated to attain thethermolysis of the binder, the pyrolysis of the carbon source, and thedecomposition of the dihydroxybis (ammonium lactate) titanium (IV)) toleave a simplified titanium-bearing residue, postulated to be titania.Thereafter, the thermolized and pyrolized green body is subjected topressureless sintering to obtain a pressureless sintered body, thepressureless sintering step optionally including an intermediate thermalstep to ensure volatilization of boron oxide coatings of the boroncarbide particles in the powder. The pressureless sintered body is thencooled, and the cooled pressureless sintered body is HIPed.

According to yet another aspect of the present invention, a post-HIPedbody is obtained which includes boron carbide, and titanium diboride.The body so obtained is suitable for applications requiring highhardness, for example, armor. Furthermore, a body so obtained can have amedian grain size distribution approaching a value less than one micron,i.e. closely approaching the median size distribution of the startingboron carbide powder.

A process according to the preferred embodiment of the present inventionuses a boron carbide powder with a narrow particle size distribution aspart of a methodology to produce a body having no porosity and small(sub-micron) average grain size, resulting in Vickers hardness valueshigher than values previously obtained for boron carbide bodies madewith a boron carbide powder of wider particle size distributions and noadded impurities. In the preferred embodiment, carbon additions in theform of water-soluble phenolic resin, and titanium additions in the formof dihydroxybis (ammonium lactato) titanium (IV), introduced in thepre-spray drying aqueous suspension, facilitate intimate coating of theadditives on the boron carbide particles in spray dried granules. Afterthermolysis/pyrolysis heat-treatment of uniaxially and coldisostatically pressed bodies made with the coated boron carbideparticles (powder composition), graphite and titanium-containing phases(postulated to be a B₂O₃—TiO₂ eutectic liquid) coat the B₄C particles ofthe green compact. During sintering heat-treatment, the graphite isinterpreted to have reacted away boron oxide coatings on the B₄Cparticles, facilitating a lower temperature onset of sintering, alongwith functioning as a sintering aid and grain growth inhibitor at highertemperatures. At these higher temperatures, the titanium-bearing phasereacts with graphite and boron carbide to form titanium diboride,depleting the concentration of free graphite in the sintered andpost-HIPed body. The extent of grain growth is attenuated by thecombination of 1) starting with powder of narrow particle sizedistribution, 2) the presence of the added impurities, pinning grainboundary movement, and 3) the use of the lowest sintering soaktemperatures which sinter the body to a closed-porosity state (asopposed to sintering to the maximum achievable relative density). Theresulting microstructure of samples prepared through a process accordingto the preferred embodiment yielded higher post-HIPed relativedensities, smaller grain size, and lesser graphite grain coarsening. Thecombination of the two additives, intimately mixed with B₄C powder,allows the use of graphite additions which facilitate densification, butwhose (hardness-reducing) concentration is attenuated in the latterstages of sintering through the process of forming a very hard TiB₂phase. Thus, a process for fabricating a ceramic body according to thepresent invention includes preparing a powder composition that includesboron carbide particles coated with a titanium compound; forming a greenbody from the powder composition; and pressureless sintering the greenbody to obtain a sintered boron carbide body having closed porosity. Thepowder composition is prepared by preparing a slurry from the boroncarbide particles and a titanium containing solution, and spray dryingthe slurry. The slurry is preferably water-based but can be preparedwith any suitable solvent. The titanium containing solution includes anorganometallic titanium compound that is capable of adhering to theexterior surfaces of the boron carbide particles so that it may coat theparticles. A process according to the present invention can produce asolid article that includes boron carbide phases and titanium diboridephases.

Solid articles of higher hardness can be produced by adding a carboncontaining compound to the slurry. In the preferred embodiment, theorganometallic compound and the carbon containing compound are watersoluble and the slurry is prepared by mixing the organometalliccompound, the carbon containing compound and the boron carbide particlesin water. In the preferred embodiments, the organometallic compound isammonium lactato titanium (IV) and the carbon containing compound isphenolic resin.

A solid article according to the present invention is formed from aboron carbide powder that includes boron carbide particles of a givenmedian size, the solid article including a sintered body that includes aboron carbide phase and a titanium diboride phase, the phases beingpresent in granular form, wherein the relative density of the sinteredbody is more than 99% and the hardness of the sintered body is more than2600 Vickers, and the median grain sizes of the grains of each phase isno more than 100% larger than the median size of the boron carbideparticles in the starting powder. The sintered body may further includea carbon-based phase, wherein the carbon-based phase is 0-0.5% of thevolume of the sintered body. In the preferred embodiments, the sinteredbody includes 0.081-5 wt % titanium in the form of TiB₂. Also, in thepreferred embodiments, up to 50% of grains of the phases are less than 1μm in diameter, and more preferably up to 40% of grains of said phasesare less than 0.5 μm in diameter.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A shows the steps in a process for the fabrication of a boroncarbide body according to the present invention.

FIG. 1B shows the steps for the preparation of a granulated powdercomposition according to the present invention.

FIG. 1C illustrates a number of boron carbide particles preparedaccording to the process shown in FIG. 1B, coated with atitanium-containing compound.

FIG. 1D illustrates a number of boron carbide particles preparedaccording to the process shown in FIG. 1B coated with atitanium-containing compound and a carbon-containing compound.

FIGS. 2A, 2B and 2C graphically show relative densities of pressurelesssintered samples prepared from powder compositions containing 0.081,0.163, 0.488, 0.814 wt % titanium and 0 wt % carbon (FIG. 2A), 1.2 wt %carbon (FIG. 2B) and 3.7 wt % carbon (FIG. 2C), as a function ofsintering soak temperature, determined by the Archimedes method, usingthe rule of mixtures of as-batched compositions as the basis fortheoretical densities. Numbers labeling various traces indicate theweight percentage of titanium (on an as-batched basis).

FIGS. 3A, 3B and 3C graphically show relative densities of pressurelesssintered and post-HIPed samples of FIGS. 2A, 2B and 2C respectively as afunction of sintering soak temperature (HIP schedule identical for alldata). Numbers labeling various traces indicate the weight percentage oftitanium (on an as-batched basis).

FIGS. 4A and 4B graphically show hardness values of HIPed samples whichwere exposed to the two lowest pressureless sintering soak temperatures(2240° C., FIG. 4A, 2260° C. FIG. 4B) during pressureless sinteringheat-treatments.

FIGS. 5A, 5B and 5C graphically show relative densities of pressurelesssintered samples prepared from powder compositions containing 0.0, 1.33,2.40 wt % titanium and 0 wt % carbon (FIG. 3A), 1.2 wt % carbon (FIG.3B) and 3.7 wt % carbon (FIG. 3C), as a function of sintering soaktemperature, determined by the Archimedes method, using the rule ofmixtures of as-batched compositions as the basis for theoreticaldensities. Numbers labeling various traces indicate the weightpercentage of titanium (on an as-batched basis).

FIGS. 6A, 6B, and 6C graphically show relative densities of samples ofFIGS. 5A, 5B and 5C respectively after post-HIPing as a function ofsintering soak temperature. Numbers labeling various traces indicate theweight percentage of titanium (on an as-batched basis).

FIGS. 7A, 7B, and 7C graphically show Vickers hardness values for theHIPed samples of FIGS. 6A, 6B and 6C respectively as a function ofpressureless sintering soak temperatures. For comparison, the Vickershardness of PAD-B₄C (hot-pressed, BAE Systems, 2632.4□111.7) is shown asa dashed line measured on the same instrument using the same parametersand methodology used for measuring the hardness of the samples.

FIG. 8 shows an optical micrograph of HIPed sample prepared from apowder composition containing 1.33 wt % Ti and 3.7 wt % C, which wasexposed to a pressureless sintering soak temperature of 2200° C.

FIG. 9 graphically shows cumulative percent number finer plots based onthe line-intercept method analysis of optical micrographs of polishedsections of samples prepared from a powder composition containing 1.33wt % Ti, 3.7 wt % C. Median grain size (d₅₀) values are indicated by thedashed lines.

FIGS. 10A, 10B and 10C graphically show median grain sizes determinedfrom cumulative percent finer plots (d₅₀) generated from image analyses(linear intercept method) of optical micrographs of selected samplesprepared according to the present invention.

FIGS. 11A and 11B graphically show HIPed relative density and hardnessvalues for the samples of FIGS. 3A-3C and 6A-6C.

FIGS. 12A, 12B, 12C and 12D show optical micrographs of sintered andpost-HIPed samples prepared with powder composition containing 1.33 wt %Ti and 3.7 wt % C, soaked during pressureless sintering at 2200° C.(FIG. 12A), 2220° C. (FIG. 12B), 2240° C. (FIG. 12C) and 2260° C. (FIG.12D).

FIG. 13 shows an SEM micrograph of a polished specimen cross sectionprepared from a powder composition containing 2.40 wt % Ti and 1.2 wt %C, sintered at a pressureless sintering soaking temperature of 2220° C.and then post-HIPed.

FIGS. 14A, 14B, 14C and 14D show optical micrographs of polished/etchedsections of pressureless sintered and HIPed specimens prepared frompowder compositions containing 0.018 (FIG. 14A), 0.163 (FIG. 14B), 0.488(FIG. 14C), and 0.814 (FIG. 14D) wt % titanium and 3.7 wt % carbon,pressureless sintered at 2240° C.

FIGS. 15A, 15B and 15C show optical micrographs of polished/etchedsections of sintered and HIPed samples prepared from powder compositionscontaining 0 (FIG. 15A), 1.33 (FIG. 15B), and 2.4 (FIG. 15C) wt %titanium and 3.7 wt % carbon, pressureless sintered at 2200° C.

FIG. 16: graphically shows area fractions (percent) of graphite based ondigital analysis of the micrographs in FIGS. 14A-14C and 15A-15C (3.7 wt% C). The horizontal dashed line indicates the 3.7 wt % C, whichtranslates to 4.29 vol % C.

FIG. 17 shows X-ray diffraction patterns of as-received boron carbidepowder as well as ground and polished surfaces of two sintered and HIPedsamples prepared from a powder composition having 2.4 wt % Ti, 1.2 wt %C, and pressureless sintered at 2220° C., and another powder compositionhaving 1.3 wt % Ti, 3.7% carbon and pressureless sintered at 2200° C.

FIG. 18A shows XRD patterns of green compact specimens (different lotnumber of Starck HD15 powder than other data) prepared from powdercompositions composed of 3.7 wt % C and 1.33 wt % Ti additions, heatedat 15° C./min to the indicated temperature, and then furnace cooled. B:B₄C, U: Unidentified phase, G: Graphite, O: B₂O₃, T: TiB₂, nC: carbonnanotubes.

FIG. 18B shows a magnified view (restricted 2θ range) of FIG. 18A.

FIGS. 19A, 19B and 19C graphically show relative density values forsamples prepared from a powder composition containing 0.5, 1.0, 3.0, 5.0wt % Ti, and 0 (FIG. 19A), 1.2 (FIG. 19B) and 3.7 (FIG. 19C) wt %carbon, after sintering heat-treatment, as a function of sintering soaktemperature, the titanium source being 0.9 μm TiO₂ particles.

FIGS. 20A, 20B and 20C graphically show relative density values of thesamples of FIGS. 19A, 19B and 19C respectively after HIPing, as afunction of sintering soak temperature.

FIG. 21 graphically shows Vickers hardness values of selected samplesfrom the samples of FIGS. 20A, 20B and 20C.

FIG. 22 shows an XRD pattern of a post-HIPed sample prepared from apowder composition containing 0.9 μm TiO₂ and phenolic resin additions,yielding 5 wt % Ti and 3.7 wt % C, exposed to a sintering soaktemperature of 2260° C. B: B₄C, G: Graphite, T: TiB₂.

FIG. 23A shows an optical micrograph of a sample prepared from a powdercomposition prepared with 0.9 μm TiO₂ and phenolic resin additions,yielding 5 wt % Ti and 3.7 wt % C, exposed to a sintering soaktemperature of 2260° C.

FIG. 23B shows an optical micrograph of the sample prepared with thesame chemical composition as in FIG. 23A in which the titanium sourcewas 32 nm TiO₂ particles.

FIGS. 24A, 24B and 24C graphically show relative density values forsamples prepared from a powder composition containing 0.5, 1.0, 3.0, 5.0wt % Ti, and 0 (FIG. 24A), 1.2 (FIG. 24B) and 3.7 (FIG. 24C) wt %carbon, after sintering heat-treatment, as a function of sintering soaktemperature, the titanium source being 32 nm TiO₂.

FIGS. 25A, 25B and 25C graphically show relative density values for thesamples in FIGS. 24A, 24B and 24C respectively after HIPing, as afunction of sintering soak temperature. Labels in the figure correspondto Ti weight percent.

FIG. 26 graphically show Vickers hardness values of selected post-HIPedsamples of FIGS. 25A, 25B and 25C pressureless sintered at 2240° C. and2260° C., as a function of Ti content (32 nm TiO₂).

FIG. 27 graphically shows relative densities and hardnesses of samplesprepared according to the present invention with a different startingboron carbide powder (H. C. Starck HS B₄C powder), as a functionsintering soak temperature, the powder composition of the sample was1.33 wt % Ti and 3.7 wt % C.

FIG. 28 graphically shows relative densities and hardnesses of samplesprepared based on B₄C powder manufactured by U. K. Abrasives, which hada median particle size of 0.5 μm. The sintering soak temperature wasfixed at 2200° C. and the carbon content was fixed at 3.7 wt %.

FIG. 29 shows an X-ray diffraction pattern of a post-HIPed sample madefrom a powder composition containing UK Abrasives powder (d₅₀=0.5 μm),1.33 wt % Ti and 3.7 wt % C, exposed to a pressureless sintering soaktemperature of 2200° C. B: B₄C, G: graphite, T: TiB₂, S: SiC.

FIG. 30 graphically shows relative densities and hardnesses of samplesprepared according to the present invention from a powder compositionthat contained B₄C powder manufactured by U. K. Abrasives (which had amedian particle size of 1.7 μm), and 5.15 wt % carbon. The sample waspressureless sintered at 2200° C.

FIG. 31 graphically shows standard deviations in measured hardnessvalues. Data represented by circles corresponds to Table 4, while datarepresented by squares corresponds to Table 5.

DETAILED DESCRIPTION

Referring to FIG. 1A, a process according to the present invention usesa granulated powder composition which is comprised of boron carbideparticles coated at least with a titanium compound. Step S1 refers tothe process for preparing the granulated powder composition. Referringto FIG. 1B, according to the present invention, the powder compositionis prepared by first preparing a slurry that contains at least boroncarbide particles and a titanium containing solution that includes atitanium compound capable of adhering to the exterior surfaces of theboron carbide particles after drying. The slurry is then spray dried toobtain a granulated powder composition suitable for forming a compactunder pressure. FIG. 1C illustrates a powder composition according tothe present invention. As illustrated, a powder composition according tothe present invention includes boron carbide particles that are coatedat least with a titanium compound. In other embodiments, a carboncontaining compound may be added to the slurry before the slurry isspray dried. Preferably, the carbon containing compound is also capableof adhering to the exterior surfaces of the boron carbide particles.Thus, as illustrated in FIG. 1D, the powder composition may includeboron carbide particles that are coated with a titanium-containingcompound, or a titanium-containing compound and a carbon-containingcompound.

Referring to FIG. 1A, a compact (green body) is formed from thegranulated powder composition (S1) according to any suitable method(e.g. uniaxial pressing, isopressing, extrusion). The green body is thenheated (S3) to cause the thermolysis and/or pyrolysis of thetitanium-containing compound and/or carbon-containing compound in thegreen body. After the thermolysis/pyrolysis step (S3), the green body ispressureless sintered (S4) to obtain a pressureless sintered body havingclosed porosity (typically, but not always, a closed porosity boroncarbide body will have a relative density in the range of 93-96%). Thepressureless sintered body is then cooled (S5) and further densifiedthrough HIPing (S6).

First Series of Samples with Powder Compositions Having Less than 1 wt %of Titanium

The first series of samples, which were prepared according to theprocess disclosed in FIG. 1A and described above, were boron carbidebased bodies that included titanium diboride (TiB₂). The starting powdercomposition for the preparation of samples included less than 1 wt % oftitanium and were prepared according to the process disclosed in FIG. 1Band described above. The boron carbide powder used to prepare the firstseries of samples (and the second series of samples disclosed below) wasa high purity boron carbide powder (HD15, H. C. Starck, GmbH, Goslar,Germany) of narrow particle size distribution (d₁₀-d₉₀=0.2-1.5 μm with ad₅₀=0.6 μm). The characteristics of the boron carbide powder used forthe preparation of the first and the second series of samples is setforth in Table 1.

TABLE 1 Boron Carbide Powder Characteristics (HD15) Specific Surfacearea (m²/g) 19 Particle size (μm) d₉₀ = 1.5, d₅₀ = 0.6, d₁₀ = 0.2Impurity Content (wt %) 1.7 O, 0.2 N, 0.09 Si, 0.0 Fe, 0.02 Al CarbonContent (wt %) 21.8 B:C Ratio 3.9

In each case, to perform powder processing (S1, FIG. 1A) to obtain agranulated powder having coated boron carbide particles, slurries wereprepared by mixing in deionized water, batches containing 200 g of boroncarbide powder, a water-soluble titanium organometallic-containingsolution (titanium source), a water-soluble phenolic resin (the carbonsource in samples that included carbon), binder components, and 10 dropsof a concentrated defoaming agent (Hercules Inc., Wilmington, Del.) in 2liter high-density polyethylene (HDPE) mixing jars. The titaniumorganometallic-containing solution used in each slurry was an aqueoussolution (pH 7-8) with 50 wt % of dihydroxybis (ammonium lactato)titanium (IV), C₆H₁₀O₈Ti.2(NH₄) (the titanium compound), synonym: lacticacid titanium chelate ammonium salt (Alfa Aesar, Ward Hill, Mass.). Thecarbon source (for samples that included carbon) was a water-solublephenolic resin (SP-6877, SI Group, Schenectady, N.Y.). Experimentsshowed that the phenolic resin produced 37.63 wt % carbon char followingpyrolysis at 1000° C. in flowing Ar. In each slurry, a standard bindersystem composed of 1 wt % polyvinyl alcohol (PVA), 0.5 wt % polyethyleneglycol (PEG) plasticizer, and 1 wt % Darvan 821A dispersant (R.T.Vanderbilt Company, Norwalk, Conn.) was used as a water-soluble slurryadditives.

The samples of the first series were prepared based on powdercompositions in which the content of the titaniumorganometallic-containing solution in the slurries were selected toproduce a powder composition after drying that included 0.081 wt %,0.163 wt %, 0.488 wt %, and 0.814 wt % of titanium by weight. Additionsof 0.5, 1, 3, and 5 wt % of the organometallic Ti additive correspond to0.081, 0.163, 0.488, and 0.814 wt % of elemental titanium, respectively(based on molar mass ratio of Ti to the titanium-containingC₆H₁₀O₈Ti.2(NH₄), and assuming complete conversion, i.e. 100% yield).Weight percentages of Ti of 0.081, 0.163, 0.488, and 0.814 translate toweight percentages of TiB₂ of 0.12, 0.24, 0.71, and 1.18, respectivelyin the pressureless sintered sample. For each titanium concentration,carbon concentrations of 0 wt %, 1.2 wt %, and 3.7 wt % were used.Weight percentages were determined on the basis of the weight fractionof the additive relative to weight fraction of boron carbide plus thatadditive alone. This was done so that for a given additive of, forexample, phenolic resin, higher concentrations of the organometallicadditive did not alter the concentration of carbon relative to boroncarbide. Thus, the powder compositions used for the preparation of thefirst series of samples included the following amounts (by weight) oftitanium, carbon and boron carbide powder from Table 1. Table 2 showsthe wt % calculated to be yielded from the residues from theorganometallic and phenolic resin after thermolysis/pyrolysis. Thebinder was not factored into the ratios. The carbon formed will bepartially consumed in a reaction between it, titania, and boron carbide,so the listed titanium content is assumed to be in the finalmicrostructure, but the calculated carbon will be partially consumed andwill not be represented in the final microstructure.

TABLE 2 Calculated Ti Calculated Carbon Yield from the Yield BoronOrganometallic from Phenolic Carbide Additive (wt %) Resin (wt %) Powder0.081   0% Balance 0.081 1.2% Balance 0.081 3.7% Balance 0.163   0%Balance 0.163 1.2% Balance 0.163 3.7% Balance 0.488   0% Balance 0.4881.2% Balance 0.488 3.7% Balance 0.814   0% Balance 0.814 1.2% Balance0.814 3.7% Balance

To prepare each powder composition shown in Table 2, the solids loadingof the slurries were maintained at 17-22 vol %. Slurries were mixed for24 hours in a ball mill with boron carbide media. The slurries were thenspray dried into granulated powder. During operation, the inlet andoutlet temperatures of the spray drying chamber were 260-270° C. and50-70° C., respectively. The spray drying in each case resulted in apowder composition that included spherical granules of boron carbideparticles coated with the titanium-containing compound and, in samplesthat included carbon, coated with the carbon containing compound(resulting from dried soluble phenolic resin).

To prepare a green body (S2, FIG. 1A) from each powder composition, eachgranulated powder composition was uniaxially pressed into cylindricaldisks measuring 12.7 mm in diameter and ˜3 mm in height using a toolsteel die under 150 MPa of pressure in a hydraulic hand press. All diskswere then placed into latex bags, which were subsequently evacuatedusing a mechanical vacuum pump and the bags were then sealed. The disksin the latex bags were cold isostatically pressed (CIP, AmericanIsostatic Presses, Inc., Columbus, Ohio) in a water/oil mixture at 345MPa for 2 minutes in order to improve green density and mitigate anyparticle packing density gradients resulting from the previous uniaxialpressing step. After CIPing, the weight and caliper-measured dimensionsof the disks were measured. The samples were then placed in tieredgraphite crucibles, which were in turn placed in a laboratory vacuumfurnace with tungsten heating elements and interior walls, enclosed in awater-cooled steel housing (BREW, Thermal Technology Inc., Santa Rosa,Calif.). Under continuous mechanical vacuum pumping, the compacts wereheated at 0.5° C./min from room temperature to 500° C. The samples wereheld at 500° C. for seven hours to thermolyze the binder. They were thenheated at 3° C./min to 1350° C., and held at that temperature for fourhours in order to pyrolize the phenolic resin, decompose the titaniumcompound to leave a titanium-containing residue, and allow theseconstituents to react as set forth below. The weight of each disk wasagain measured to determine the weight loss resulting from thisheat-treatment.

After the thermolysis/pyrolysis heat-treatment (S3, FIG. 1A) andsubsequent cooling, the samples were then pressureless sintered in alaboratory vacuum furnace with graphite heating elements and insulation(Thermal Technology, Santa Rosa, Calif.), and a water-cooled aluminumhousing. To pressureless sinter the green samples, the followingprocedure was followed. Following sample placement, the furnace chamberwas evacuated to a pressure of less than 80 mTorr using a mechanicalvacuum pump. The furnace was backfilled with He gas to atmosphericpressure, followed by a constant flow rate of this gas of 1 lpm, withthe exit gas bubbled through oil. The operational temperature range ofthe infrared pyrometer used for this furnace was 600-3000° C.; initialheating was based on linearly increasing furnace power until 600° C. wasreached, at which point a proportional-integral-derivative temperaturecontrol algorithm took over. This linear power increase resulted in anaverage heating rate of ˜63° C./min. After cooling, green samples wereheated at 15° C./min from 600° C. to 1300° C. and held at 1300° C. fortwo hours in order to volatilize any remaining boron oxide coatings onboron carbide particles. The samples were then heated at 15° C./min upto one of the following soak temperatures: 2240° C., 2260° C., 2280° C.,and 2300° C. Each sample was held at its respective selected soaktemperature for 30 minutes to allow for pressureless sintering. Thesamples were then cooled at the natural cooling rate of the furnace withthe heating elements turned off. An average rate of cooling of ˜60°C./min is estimated, based upon the time taken for the furnace to coolfrom 2300 to 600° C. (though initial cooling is more rapid, whilecooling as room temperature is approached is less rapid). Followingpressureless sintering, the Archimedes densities of all the samples weremeasured, and the weight loss resulting from sintering was alsorecorded.

All of the samples were then HIPed. The HIP was pressurized to 12 kpsiwhile heating to 450° C. at 15° C./min, and then heated to 1700° C. at15° C./min while at that pressure. During heating from 1700 to 2050° C.,pressure was increased to 30 kpsi, and this pressure and temperaturewere maintained for a one hour soak. The HIP was then cooled to roomtemperature with the furnace elements off; when the furnace reached˜150° C., the pressure of the chamber from cooling the gas reduced to˜7.5 kpsi; this pressure was then released to ambient pressure. AfterHIPing, the Archimedes density was again measured.

The theoretical density values used to calculate the relative densitiesof the samples were determined using the rule of mixtures based onas-batched compositions:ρ_(composite)=ρ_(B4C) V _(B4C)+ρ_(TiB2) V _(TiB2)+ρ_(C) V _(C)where ρ and V represent the theoretical density and volume fraction ofeach phase (B₄C, TiB₂, and graphite) in the microstructure,respectively. The theoretical density values used for B₄C, TiB₂, andgraphite were 2.52, 4.50, and 2.16 g/cm³, respectively. Volume fractionswere determined from converting masses using, for example,V _(C)=(m _(C)/ρ_(C))/(m _(C)/ρ_(C) +m _(B4C)/ρ_(B4C) +m_(TiB2)/ρ_(TiB2))where m is mass of a particular component. The mass of TiB₂ was based onequimolar quantities of TiB₂ forming from the added Ti.Second Series of Samples with Powder Compositions Having More than 1 wt% of Titanium

A second series of test samples were prepared using powder compositionsthat included more than 1 wt % of titanium. Specifically, powdercompositions were prepared with Ti concentrations of 1.33 wt % and 2.40wt %—selected as incremental increases over the maximum of 0.814 wt % Tiexamined in the first series. Assuming complete conversion of thetitanium provided from thermal decomposition of C₆H₁₀O₈Ti.2(NH₄), weightpercentages of Ti of 1.33 wt % and 2.40 wt % translate to weightpercentages of TiB₂ of 1.92 and 3.45, respectively. In preparing thepowder compositions for the second series of samples, the same boroncarbide powder as the one used for the first series of samples was used.Also, the same water-soluble phenolic resin was used to yield 0, 1.2,and 3.7 wt % C, as well as the other aforementioned organic additives.The samples in this second series were prepared in the same manner asthe first series with the exception that thicker green disks of ˜9 mm inheight (still 12.7 mm diameter) were prepared. Table 3 discloses thecontents of the powder composition used to prepare the samples for thesecond series. Table 3 shows the wt % calculated to be yielded from theresidues from the organometallic and phenolic resin afterthermolysis/pyrolysis. The binder was not factored into the ratios. Thecarbon formed will be partially consumed in a reaction between it,titania, and boron carbide, so the listed titanium content is assumed tobe in the final microstructure, but the calculated carbon will bepartially consumed and will not be represented in the finalmicrostructure.

TABLE 3 Calculated Ti Calculated Yield from Carbon Yield Boron theOrganometallic from Phenolic Carbide Additive (wt %) Resin (wt %) Powder0 1.2 Balance 0 3.7 Balance 1.33 0 Balance 1.33 1.2 Balance 1.33 3.7Balance 2.4 0 Balance 2.4 1.2 Balance 2.4 3.7 BalanceTesting of the Samples

The data reported herein were based on tests performed according to theprocedures set forth in this section. To characterize post-HIPed samplesfor hardness and microstructure, the samples were encapsulated inSpeciFix resin (Struers, Inc., Westlake, Ohio), and the outergraphite-rich surface was ground away using a 220 grit diamond-coatedgrinding plate, as well as metal-bonded diamond media plates combinedwith 45, 15, and 9 μm diamond suspensions (Struers Piano, Struers, Inc.,Westlake, Ohio, as well as Apex Band Metadi Supreme, Buehler, LakeBluff, Ill.). Samples were washed with deionized water following eachgrinding step. After grinding to flat surfaces, the samples werepolished on specialized cloths with 9, 3, and 1 μm polycrystallinediamond suspensions (Struers MD, Struers, Inc., Westlake, Ohio, andTexmet and Metadi Supreme, Buehler, Lake Bluff, Ill.). After eachpolishing step, the samples were washed and placed in an ultrasonicatingbath to remove residual diamond particles from the polished surfaces.

Polished surfaces were indented at arbitrary and unbiasedmicrostructural locations using a Vickers diamond indenter (Duramin-2,Struers, Westlake, Ohio, USA) under an applied load of 1 kg for 15seconds. Hardness measurements were calibrated using the SRM 2831tungsten carbide standard reference disk (SRM-2831, National Institutefor Standards and Technology, Gaithersburg, Md.). The diagonal lengthsof ten acceptable indentations (as determined by ASTM C 1327-99“Standard Test Method for Vickers Indentation Hardness of AdvancedCeramics”) were measured and average hardness values, as well asstandard deviations (in units of kg/mm²), were calculated.

The disks prepared for hardness were electrolytically etched in order toreveal the location of the grain boundaries. Samples were etched for˜20-30 seconds in dilute aqueous KOH (1 g of KOH in 100 mL of deionizedwater) using a current of 20 mA at 21 VDC applied through a thin Pt foilcathode. The microstructures of the HIPed samples were examined usingoptical microscopy (Olympus BX40, Olympus America, Inc., Center Valley,Pa.) and scanning electron microscopy (SEM, Model 1530 SEM, LEO ElectronMicroscopy, Inc., Oberkochen, Germany). Energy-dispersive X-rayspectroscopy (EDS, Oxford Pentafet Detector, Oxford Instruments,Oxfordshire, UK) was performed during SEM in order to identify thechemical composition of specific microstructural locations.

The optical micrographs were analyzed using the linear interceptquantitative characterization method to determine the cumulative percentfiner grain size distribution and median (d₅₀) grain size for selectsamples based upon 50 grain measurements across each of four separatemicrostructure images for a total of 200 measurements. To analyzemicrostructures for the area fraction (percentage) of carbon, a VisualBasic 4.0 program was written which read each pixel of a micrograph forits red, green, and blue intensities. For a black and white image, thesevalues are equivalent; a value of zero is black and a value of 255 iswhite. A cutoff value was designated, e.g. 30, wherein if a pixel was oflower value, it was taken to be a carbon region. These regions werecolored in over the micrograph to provide a visual verification of theregions counted. Some adjustments of the cutoff value and the contrastshown in the micrograph (using Adobe Photoshop) were required so thatthe carbon regions, and no other regions such as grain boundaries, werecounted. Once deemed correct, the ratio of carbon pixels to total pixelswas calculated to determine the area percent of the microstructure whichwas graphite.

The phases contained within as-received powder samples as well aspolished post-HIPed disk samples were identified by X-ray diffraction(X'Pert PRO Alpha-I, PANalytical, Almelo, The Netherlands), at a0.084°/s scan speed, a step size of 0.017°, and 2-theta range of 10° to85°. Soller slits corresponding to 0.04 radians were installed in theincident and diffracted x-ray beam pathways in order to produce scanswith minimal background interference relative to the intensities of thediffraction peaks. Additionally, a 10 mm mask was installed in theincident beam pathway, and a 5 mm mask was installed in the diffractedbeam pathway.

Results of Testing

Results for the First Series

Archimedes sintered relative density data for all of the samples in thefirst series are shown in FIGS. 2A-2C, which indicate that relativedensities after pressureless sintering increased with increasing carboncontent. The highest relative densities were generally obtained with asintering soak temperature of 2260° C. Clear trends in relative densitywith increasing Ti content were not apparent, with the exception of 0 wt% C at the lower sintering soak temperatures. Relative densities of thesamples in the first series after post-HlPing are shown in FIGS. 3A-3C.For 1.2 wt % C additions, sintering temperatures of 2280° C. and aboveyielded degraded HiPed relative densities, even when sintered relativedensities were not negatively affected. The specimen with 3.7 wt % C and0.081 wt % Ti, when sintered with a soak temperature of 2260° C., HIPedto theoretical density.

TABLE 4 Hardness of Specimens of Varying C, Ti, and Sintering SoakTemperatures (± values represent standard deviation). Sintering SoakCarbon Titanium Vickers Temperature Content Content Hardness (° C.) (wt%) (wt %) (kg/mm²) 2240 0 0.081 2198.3 ± 621.9 2240 0 0.163 2481.5 ±362.2 2240 0 0.488 2723.6 ± 166.2 2240 0 0.814 2832.0 ± 115.1 2240 1.20.081 2179.7 ± 262.3 2240 1.2 0.163 2750.3 ± 140.9 2240 1.2 0.488 2774.2± 171.0 2240 1.2 0.814 2861.8 ± 223.8 2240 3.7 0.081 2569.2 ± 184.8 22403.7 0.163 2912.3 ± 79.61 2240 3.7 0.488 2993.3 ± 79.0  2240 3.7 0.8143027.8 ± 65.8  2260 0 0.081 2247.0 ± 503.6 2260 0 0.163 2633.8 ± 225.62260 0 0.488 2668.5 ± 314.7 2260 0 0.814 2786.5 ± 159.5 2260 1.2 0.0812781.5 ± 141.8 2260 1.2 0.163 2726.5 ± 138.0 2260 1.2 0.488 2729.9 ±156.6 2260 1.2 0.814 2921.3 ± 136.0 2260 3.7 0.081 2720.0 ± 101.0 22603.7 0.163 2700.8 ± 137.9 2260 3.7 0.488 2682.9 ± 129.1 2260 3.7 0.8142742.5 ± 98.7 

Hardness values after HIPing for the first series samples are shown inTable 4 and FIGS. 4A and 4B. Note that Table 4 identifies each samplebased on the powder composition used for the preparation of the sampleand a calculated yield of titanium and carbon. Since the relativedensities of samples sintered at higher soak temperatures (i.e. 2280° C.and 2300° C.) were low, hardnesses of those specimens were not measured.Specimens with 3.7 wt % C exposed to 2240° C. soaking temperaturesachieved markedly higher hardnesses for any given Ti concentration thanthose exposed to the higher sintering soaking temperatures of 2260° C.,even though the latter specimens were generally (i.e. with the exceptionof the specimen with 0.814 wt % Ti) of higher post-HIPed relativedensity. For specimens soaked at 2240° C. during sinteringheat-treatments, increasing titanium content resulted in increasinghardness for all three (0, 1.2, 3.7 wt %) carbon contents. At 2240° C.,for a given Ti content, higher carbon concentrations yielded higherhardness values. This was not the case for the 2260° C. soakingtemperature in which 3.7 wt % C additions were of lower hardness than1.2 wt % C additions. Thus, for the first series, the highest hardnesswas obtained with the highest carbon and titanium contents, and thelowest sintering soak temperature (i.e. at 2240° C.).

Results for the Second Series

In a second series, the effect of higher concentrations of Ti and lowersintering soak temperatures than the previous study were evaluated,based on trends pointing toward the highest hardnesses in the previousstudy. In addition, to clarify the individual effect of Ti additions, a0 wt % Ti addition composition series was prepared and evaluated. FIGS.5A-5C show the relative densities achieved with these compositions afterpressureless sintering and before HIPing. As before, relative densitiesincreased with higher carbon content. In the second series, relativedensity after pressureless sintering heat-treatment generally increasedwith increasing Ti content. Relative densities of these specimens afterHlPing are shown in FIGS. 6A-6C. For 1.2 wt % C, sintering soaktemperatures above 2220° C. generally resulted in lower HIPed relativedensities, even though sintered relative density increased withincreasing sintering soak temperature.

TABLE 5 Hardness Values from the Samples in the Second Series (± valuesrepresent standard deviation) Sintering Soak Carbon Titanium TemperatureContent Content Hardness (° C.) (wt %) (wt %) (kg/mm²) 2200 3.7 0 2780.1± 70.5  2200 3.7 1.33 3137.6 ± 99.7  2200 3.7 2.40 2943.4 ± 87.9  22201.2 0 2856.2 ± 76.5  2220 1.2 2.40 3130.3 ± 82.0  2220 3.7 0 2740.6 ±141.6 2220 3.7 1.33 2709.3 ± 155.5 2220 3.7 2.40 2845.4 ± 97.5  2240 0 01956.7 ± 321.0 2240 0 1.33 2332.9 ± 308.3 2240 0 2.40 2541.6 ± 229.72240 1.2 0 2579.0 ± 126.9 2240 1.2 1.33 2724.9 ± 175.4 2240 1.2 2.402776.3 ± 179.3 2240 3.7 0 2504.0 ± 163.3 2240 3.7 1.33 2620.0 ± 216.32240 3.7 2.40 2692.7 ± 150.7 2260 0 0 2351.7 ± 229.1 2260 0 1.33 2269.9± 319.8 2260 0 2.40 2339.2 ± 279.7 2260 1.2 0 2282.2 ± 186.7 2260 1.21.33 2488.7 ± 259.4 2260 1.2 2.40 2439.1 ± 229.5 2260 3.7 0 2219.4 ±285.2 2260 3.7 1.33 2528.4 ± 115.0 2260 3.7 2.40 2585.8 ± 158.5

Note that the samples in Table 5 are identified based on the powdercomposition used (see Table 3). Vickers hardness values for thesepost-HIPed samples are shown in Table 5 and FIGS. 7A-7C. Specimens oflower HIPed density were not evaluated for hardness. For specimens withno added carbon, the 2240° C. sintering soak temperature yielded sampleswhose hardness increased with relative density (increasing Ti content),as expected. For these compositions soaked at 2260° C., the hardnessesand HIPed relative densities of the various Ti-content specimens merged.Hardnesses were generally higher for the 1.2 and 3.7 wt % C specimens ascompared to specimens with no added carbon. For these carbon-containingspecimens, lower sintering soak temperatures resulted in higherhardnesses, which corresponds to increasing HIPed relative densities.For the 2220-2240° C. range, for a given sintering soak temperature,hardness decreased when increasing the carbon content from 1.2 to 3.7 wt%, even though relative density increased. The 3.7 wt % C, 1.33 wt % Tispecimen exposed to a sintering soak temperature of 2200° C., and the1.2 wt % C, 2.40 wt % Ti specimen with a sintering soak temperature of2220° C., reached the highest attained hardnesses, in fact achievingvalues higher than pressured assisted densified (PAD) boron carbide,which is the benchmark value commonly used to assess the hardness of aboron carbide article. FIGS. 7A-7C include a dashed line intended toshow the hardness value of a PAD boron carbide, which is typicallyaround 2600 Vickers.

Microstructure

An example optical micrograph of a polished and electrolytically-etched(which exposed the grain boundaries) specimen cross-section surface isshown in FIG. 8. Dark regions are presumed to be graphite, graphiterich, or pores from which graphite was removed in the polishing process.Light-shaded regions are titanium diboride. Cumulative percent numberfiner (CPNF) plots were generated from quantitative analyses of suchmicrographs (e.g. FIG. 9) for selected samples from the first and secondseries. Median grain diameters (d₅₀, i.e. the 50% points in CPNF plots)for samples made with powder compositions containing 3.7 wt % C aredisplayed in FIGS. 10A-10C. In FIG. 10A, grain size increased withincreasing titanium content for a sintering soak temperature of 2260° C.Grain size was finer and more insensitive to titanium content for thesintering soak temperature of 2240° C. Correspondingly, the hardnesseswere higher for the finer grain-sized specimens exposed to the lowersintering soak temperature (FIG. 4A). An apparent inconsistency is seenin the results of the second series shown in FIG. 10B, in that asintering soak temperature of 2220° C. resulted in larger grains thanthe soak temperature of 2240° C. used in the first series (FIG. 10A).However, generally, grain sizes for samples made with powdercompositions containing 3.7 wt % carbon remained small, resulting in ahigher hardness values, with decreasing sintering soak temperature.

The density and hardness data for samples made with powder compositionsthat contained 3.7 wt % C in the first series (FIGS. 2A-2C and 3A-3C andFIGS. 4A-4B) were re-plotted in a different way in FIG. 11A. It isapparent that while the higher sintering soak temperature of 2260° C.yielded generally higher HIPed relative densities than those exposed tothe lower soak temperature of 2240° C., the hardnesses of the less densespecimens were actually higher. The differentiating feature, beyondrelative density, for specimens exposed to these two sintering soaktemperatures, is the median grain sizes shown in FIG. 10A; grain sizeswere significantly smaller for the lower sintering soak temperature(harder) specimens.

The samples made with a powder composition that included 3.7 wt % C and1.33 wt % Ti, exposed to a sintering soak temperature of 2200° C., had aremarkably small d₅₀ grain size of 0.84 μm (FIG. 10B), which is onlyabout 40% larger than the d₅₀ particle size of 0.6 μm of the boroncarbide powder used to prepare the powder composition. Typically, themedian grain size of a sintered boron carbide article made with boroncarbide powder is several times larger than the median grain size of theboron carbide powder used to prepare the article because of the graingrowth which occurs during the final stages of sintering. However,samples prepared according to the present invention showed median grainsizes, which were well less than 100% larger than the median grain sizeof the boron carbide powder used in their preparation, whichadvantageously led to the higher hardness values reported herein. Forthis composition exposed to various sintering soak temperatures (FIG.10D), an increase in median grain size of an order of magnitude isobserved when increasing the pressureless sintering soak temperaturefrom 2200° C. to 2240° C. As shown in FIGS. 12A-12D, over this sinteringsoak temperature range, substantial coarsening of (dark) graphiteregions occurred. There is no visual indication of coarsening ofTi-containing (light-shaded, TiB₂) grains. The SEM micrograph in FIG. 13shows distinct light-shaded sharp-facet grains of TiB₂ in a B₄C matrix.The grain boundaries between B₄C grains show no visibly distinct secondphases. FIGS. 14A-14D and 15A-15D show the changes in microstructurewith increasing Ti content for a sample made with a powder compositionthat included 3.7 wt % C with sintering soak temperatures of 2240 and2200° C., respectively. For both sintering soak temperatures, initialincreases in Ti resulted in a substantial decrease in size of thegraphite grains. FIG. 16 graphically illustrates the results of aquantitative analysis of the micrographs to determine the percentage ofthe area occupied by the carbon containing phase, namely graphite. Tiadditions of 0.163 wt % or more decreased the volume percent of graphiteto values below those which were added via phenolic resin (the reductionin volume percentage is being assumed to be based on the reduction inthe area occupied by the carbon containing phase (graphite)). With theexception of the sample prepared from a powder composition that included2.4 wt % Ti and exposed to a soak temperature of 2200° C., this trend ofdecreasing graphite concentration in the microstructure with increasingTi additive corresponds to an increase in hardness.

XRD patterns were taken for the two compositions yielding the highesthardnesses (prepared from a powder composition containing 1.33 wt % Ti,3.7 wt % C and pressureless sintered at 2200° C., and prepared from apowder composition containing 2.4 wt % Ti, 1.2 wt % C and pressurelesssintered at 2220° C.) in order to determine the phases present (FIG.17). The integrated intensity of the most intense TiB₂ diffraction peak(44.93° 2θ) increased with added Ti content, as expected. The integratedintensity of the most intense diffraction peak for graphite at 26.426°2θ followed the trend of increasing with the amount of added carbon.Remarkably, the integrated intensity for graphite-3R in the as-receivedB₄C powder was greater than the two sintered samples to which wereprepared from powder compositions with added carbon (along withtitanium).

Third Series of Samples Prepared with Different Boron Carbide Powders

To evaluate the effect of the use of the water-soluble C₆H₁₀O₈Ti.2(NH₄)additive and phenolic resin on different boron carbide powder sources, adifferent grade of powder from H.C. Starck, Starck HS powder, along withtwo powders from UK Abarasives (Northbrook, Ill.), with median particlesizes of 0.5 and 1.7 μm, were evaluated. The process for the preparationof samples for this comparative study was the same as the processdescribed above for the first series of samples except that the boroncarbide powder source was different.

Use of C₆H₁₀O₈Ti.2(NH₄) and Phenolic Resin with H. C. Starck HS B₄CPowder

To evaluate the efficacy of a method according to the present inventionwith different powder sources, a composition of 1.33 wt % Ti (viaC₆H₁₀O₈Ti.2(NH₄)) 3.7 wt % C (via phenolic resin) was used with adifferent powder, i.e. H. C. Starck HS B₄C powder (See Table 5 for thecharacteristics of the powder). This powder, had a slightly highermedian particle size, and a broader particle size distribution than theH. C. Starck HD B₄C described previously (See Table 1). The HS B₄C isalso a significantly more expensive powder.

TABLE 5 Boron Carbide Powder Characteristics (HS) Specific Surface area(m²/g) 18.8 Particle size (μm) d₉₀ = 2.99, d₅₀ = 0.84, d₁₀ = 0.24Impurity Content (wt %) 1.5 O, 0.41 N, 0.09 Si, 0.022 Fe, 0.055 Si,0.003 Al, 0.23 other Carbon Content (wt %) 22.26 B:C Ratio 3.76

FIG. 27 discloses the results obtained by preparing sample from the HSpowder. The trends shown in FIG. 27 are similar to those observed forthe HD powder; sintering soak temperatures at and above 2160° C. reacheda closed porosity state in which HIPing brought the compacts close totheoretical density. The highest hardness was obtained for the 2160° C.soak temperature (3004.8 kg/mm²), with hardness dropping with increasingsintering soak temperature. The broader particle size distribution ofthis powder facilitated a higher green density (geometric relativedensity after 1300° C. thermolysis heat treatment: 78-79%) than observedfor HD (62-72%), in turn allowing for lower sintering soak temperaturesto reach the closed porosity state. However, the highest hardnessmeasured for this powder (to the limit of the experiments performed) waslower than the highest obtained for B₄C HD powder. Nevertheless, theexperiment indicates that the method disclosed herein can result inattaining a high hardness value when a different boron carbide powder isused as the starting material.

Use of C₆H₁₀O₈Ti.2(NH₄) and Phenolic Resin with Impure or RelativelyCoarse B₄C Powder

B₄C powder was obtained from UK Abrasives (Northbrook, Ill.), which hada manufacturer's specified median particle size of 0.5 μm. It was mixedwith 3.7 wt % C and varying concentrations of Ti, using a fixedsintering soak temperature of 2200° C. FIG. 28 shows the relativedensities of samples prepared using this boron carbide powder as astarting material. As shown in FIG. 28, relative densities were quitehigh. Note that relatively high hardness was reached with Ti content ofaround 2.4 wt %. A maximum hardness value of 2957 kg/mm² was reachedwith a Ti content of 4 wt %. The relative densities of over 100%, forboth sintered and post-HIPed specimens, comes from the use of pure boroncarbide as the basis for the theoretical density in relative densitycalculations; these powders contained a substantial amount of SiC, asshown in the XRD pattern in FIG. 29. Even with the presence of SiCnotwithstanding, the results obtain support the conclusion that a methodaccording to the present invention can be effective in attaining a highhardness value with a different boron carbide powder as a startingmaterial (i.e. with impure sources of sub-micron d₅₀ boron carbide).

To prepare another set of samples, boron carbide powder with a medianparticle size of 1.7 μm, obtained from the same source (UK Abrasives)was prepared according to the present invention with a powdercomposition containing 5.51 wt % C, various concentrations of Ti, andexposed to a sintering soak temperature of 2200° C. The results aredisclosed in FIG. 30. As shown in FIG. 30, high HIPed relative densitiescould be obtained with relatively lower sintered relative densities(e.g. 93.8%). The data in FIG. 30 indicate that hardness valuesdecreased with increasing Ti content. A hardness of 2560.2 kg/mm² for asample made with a powder composition containing 1.33 wt % Ti wassubstantially lower than obtained in the best case with theorganometallic additive to B₄C HD15 powder. However, this hardness is inline with that of the PAD-B₄C, the hardness for which is a benchmarkvalue (see dashed line FIGS. 7A-7C). Since coarser powder such as this1.7 μm (d₅₀) powder is substantially cheaper than sub-micron powder, theresults obtained indicate that a method according to the presentinvention would allow for the densification of such cost-competitivepowders into articles of use for many applications requiring highhardness.

The results of the studies with coarser and less expensive boron carbidepowders indicate that a method according to the present invention may beused to prepare a powder composition that can be formed into a greenbody, pressureless sintered and HIPed to obtain high hardness values.

Experiments to Elucidate the Effect of Coatings

To determine the effect of coating boron carbide particles with atitanium containing coating, the evolution of phases during the heattreatment was studied. To study the evolution of phases, boron carbidepellets with the additive containing 1.33 wt % Ti and 3.7 wt % C wereuniaxially pressed into 12.7 mm diameter cylinders at 181 MPa and thencold isostatically pressed at 345 MPa. It is important to note that thelot number of the HD15 (H.C. Starck, GmBH) used for this particularstudy was different than the lot number for all other studies using HD15in this disclosure. The green parts were placed in a graphite crucibleinside in a laboratory graphite furnace. The furnace was evacuated usinga mechanical vacuum pump to less than 80 mTorr and then back-filled withpure He gas to atmospheric pressure. The gas pressure was maintained atatmospheric pressure via a two-flask bubbler setup. The parts wereheated at 15° C./min to a given temperature and then cooled with asetpoint cooling rate of 100° C./min, which effectively shut the furnaceheating elements off after a couple of minutes, and the parts cooled atthe natural cooling rate of the furnace. The selected quenchtemperatures were 600, 800, 1000, 1300, 1600, 1900, and 2200° C. A finalquench experiment was performed in which a pellet was heated at 15°C./min to 2200° C. and held for 30 minutes to simulate typical sinteringconditions. Phase identification was performed by X-ray diffraction asdescribed above.

FIG. 18A shows XRD results for specimens heated to and quenched fromvarious temperatures. Based on comparing green-state XRD patterns of thetwo lot numbers, the B₄C HD powder lot in FIG. 18A shows significantlyless carbon than the B₄C HD powder lot represented in FIG. 17. FIG. 18Bis a magnified view over a more restricted 2θ range permitting ease ofview of graphite and TiB₂ peaks. The data indicates that the graphitepeak grew as carbon was liberated from the pyrolysis of phenolic resin.The phase labeled U corresponds to the crystalline precursor phaseprecipitated from the water-soluble C₆H₁₀O₈Ti.2(NH₄) additive. Theemergence of the U′ peaks is interpreted to be an intermediate solidresidue of the C₆H₁₀O₈Ti.2(NH₄) after partial decomposition. Thedisappearance of that phase, with no other identifiabletitanium-containing phase at 1000° C., is interpreted to indicatesolubility of the titanium-containing species into the boron oxideliquid phase residing on particle surfaces, which in other work has beenshown to persist as a coating on boron carbide particles up to1600-1800° C. under a constant heating rate. The titanium-containingphase is speculated to be titania, since in this form, it is most easilysoluble in the boria liquid (eutectic temperature 400-450° C.). Some ofthis borate liquid crystallized as opposed to becoming a glass oncooling and was identified as boron oxide in XRD patterns at 1000 and1300° C. After heating to 1300° C., TiB₂ is first detected, and itsconcentration increased with increasing quench temperature up to 2200°C. It is interpreted that as boron oxide volatilized away, theincreasingly titania-rich liquid phase reacted with graphite and boroncarbide to form TiB₂. Over the temperature range 1300-2200° C., thedevelopment of a TiB₂ phase was concurrent with the formation of a phase(identified from 1600 and 1900° C. quenches) corresponding to anano-scale carbon, and a decrease in relative intensity of the graphitepeak for the 1900° C. quench trace is apparent. While the inventors donot wish to be bound to any theory proposed herein, it appears that thedata obtained from the study of the evolution of phases suggests thatthe coatings on the boron carbide particles in the powder compositionprepared according to the present invention facilitate much moreintimate contact of reactants (C, TiO₂, and B₄C) than could be obtainedusing particulate (e.g. TiO₂, TiC, TiB₂) additions, resulting afine-grained microstructure with fine, well dispersed TiB₂ and graphitesecond phases (see Analysis of Results section).

Analysis of Results

Carbon in the form of graphite, is a well-known sintering aid forpressureless sintering of boron carbide. It is well known that carbonreacts with the boron oxide coatings on boron carbide particles andeliminates them so that the onset of sintering is substantially lowered.Carbon may facilitate “activated sintering” whereby it induces theformation of defects near the grain boundaries during sintering,providing conduits for mass diffusion. The presence of carbon along thegrain boundary can function to inhibit exaggerated grain growth whichcan otherwise trap porosity within grains (where their subsequentelimination with further heat-treatment is not feasible). The effect ofcarbon on increasing relative density are apparent from the datadisclosed in FIGS. 2A-2C and 5A-5C, which indicate that higher sinteredrelative densities were observed with increasing carbon content, andalso sintered relative densities were less sensitive to the selection ofsintering soak temperature with increasing carbon content (i.e. slopesof curves in FIG. 5 decrease with increasing carbon content); thisimplies better management of abnormal grain growth at higher sinteringsoak temperatures.

Although both sintered and post-HIPed relative densities were higherwith higher carbon additive content, hardness did not necessarilyfollow. As can be seen in FIGS. 7B and 7C for 0 wt % Ti additive,hardnesses of samples prepared with powder compositions having 3.7 wt %C are lower than those with 1.2 wt % titanium additive for any givensintering soaking temperature. Graphite has weak van der Waals bondingin one crystallographic direction; microstructural regions concentratedwith graphite would have very low hardness. Thus, an increasingconcentration of graphite in the microstructure would lower measuredhardness. The higher graphite in the samples prepared from powdercompositions having 3.7 wt % C was in fact more detrimental to hardness(FIGS. 7B and 7C) than the higher porosity (FIGS. 6B and 6C) in the 1.2wt % C specimens. This has always been the tradeoff with using carbon asa sintering aid for boron carbide. That is, carbon facilitiesdensification, but its presence in the microstructure imbues lowerhardness.

Simultaneous introduction of carbon and titanium in the form ofwater-soluble species has the distinct advantage of intimately coatingthe surfaces of all boron carbide particles in the granulated powderwith phenolic resin and a titanium-containing precursor phase. Afterheat-treatment to between 800 and 1000° C., the titanium-bearingprecursor phase remarkably disappears from XRD patterns, with nodetected Ti-containing crystalline phase taking its place (FIGS. 18A and18B). It is interpreted that the precursor phase decomposes into titaniawhich is soluble in the borate liquid phase (forming a glass aftercooling to room temperature, which cannot be detected by XRD). In the1000-1300° C. range, graphite would react away boria liquid (along withthe boria component in the liquid having an appreciable vapor pressure);the titania, enriched in the liquid phase, is interpreted to react withgraphite and B₄C according to the following reaction:3C_((s))+2TiO_(2(s))+B₄C_((s))═2TiB_(2(s))+4CO_((g))

The graphite in the microstructure would then be replaced by particlesof TiB₂ which have hardnesses competitive with boron carbide (Knoophardnesses, 100 g load: 2800 kg/mm² for B₄C, 2850 kg/mm² for TiB₂ [8]).XRD (FIG. 17) and microstructural analyses (FIG. 16), provide compellingevidence of graphite extraction because of the presence of the titaniumadditive. For the first and the second experimental series preparedbased on powder compositions containing 3.7 wt % C, hardness generallyincreased with increasing titanium content. Higher titanium contentconsumed more graphite during sintering heat treatment, attenuating theconcentration of soft-spots (graphite) in the microstructure, replacingthem with hard TiB₂ regions.

For a given composition, with increasing sintering soak temperature,graphite grains coarsened and increased in volume percent (FIG. 12). Thehigher sintering temperature facilitated preferential vaporization ofboron from boron carbide. The relative contributions of graphitecoarsening and exaggerated grain growth with increasing sintering soaktemperature to decrease in hardness with increasing sintering soaktemperature has not been differentiated.

The maximum post-HIPed relative densities (FIGS. 3A-3C and 6A-6C)resulted from the use of sintering soak temperatures lower than thosewhich yielded the highest relative densities after sintering (with nopost-HIPing). The higher soak temperatures required to raise thesintered relative densities to their maximum facilitated more extensivegrain growth, trapping more porosity within the grains where subsequentpost-HIPing could not eliminate them.

There were cases shown in which specimens with lower post-HIPed relativedensities in fact had higher hardnesses (FIG. 11A). This further showsthe advantage of low sintering soak temperatures in minimizing graingrowth; microstructures with grain sizes approaching the originalparticle sizes demonstrated the highest hardnesses. This is consistentwith Hall-Petch behavior typical in ceramics in which hardness increaseswith decreasing (square root of) grain size.

These results clearly foster the previously non-obvious conclusion thatthe lowest sintering temperature which achieves a relative density nohigher than that required to reach the closed-porosity state (requiredfor HiPing to be effective) ultimately yields the highest hardnessmicrostructures in post-HIPed samples.

Heat treatments which resulted in lower hardness also resulted ingreater deviations in measured hardness (FIG. 31). High hardnessspecimens tend to have low porosity, finely and homogeneouslydistributed second phases (graphite and TiB₂), and fine grain size. Whenthis is not the case, the indenter will interact with varyingmicrostructural features with every indent, increasing the standarddeviation in measured hardnesses. Thus the development described hereinwhich yields higher average hardnesses than previously recorded willalso yield high consistency in hardness at all locations in the article,which is a great advantage when an article fabricated according topresent invention is used in a high risk environment such as personalarmor for intercepting ballistic projectiles.

The hardnesses of the 1.33 wt % Ti, 3.7 wt % C specimen exposed to asintering soak temperature of 2200° C. along with that of the 2.40 wt %Ti, 1.2 wt % C specimen exposed to a sintering soak temperature of 2200°C. were remarkably high relative to available reported hardnesses ofboron carbide. These specimens had a grain size very close to thestarting particle size. Retaining the starting particle size in thepost-HIPed grains represents the limit of the hardness which can beachieved. The particular powder used was prepared (jet-milled) so thatit had a comparatively narrow particle size distribution; the d₉₀ to d₁₀range for this powder was 1.5 to 0.2 μm. While imbuing a comparativelypoor green density, such a distribution attenuated the driving force forgrain growth. It is expected that if a narrow particle size distributionpowder with a smaller d₅₀ were used, even higher hardnesses would beobtained.

The optimum additions of carbon and titanium using the methods describedherein will vary with the powder source; the amount of carbon needed tofacilitate sintering will likely need to be increased with increasingboron carbide particle size, and correspondingly the amount of addedtitanium will need to increase to consume much of this carbon to formhard, fine, well dispersed TiB₂ grains. Conversely, a boron carbidesource with a higher concentration of free carbon (a common impurity),will require less carbon addition. The examples given herein demonstratethe methodology to establish the additive content and sintering soaktemperatures needed to optimize hardness for differing sources of boroncarbide, or lot numbers of the same grade and manufacturer of boroncarbide.

The practical upper limit to organometallic Ti additions for the purposeof improving hardness is limited by other considerations:Thermolysis/pyrolysis heat treatments are typically performed slowly soas to not damage the part during the evolution of gases from the compactinterior. This sensitivity generally increases with increasing articlethickness. It has been found that additions of, for example, ˜4 wt % Tivia the organometallic additive results in cracking of 4″×4″ tiles of˜0.366″ thickness, using the aforementioned thermolysis/pyrolysisschedule. It is possible that an even slower schedule may overcome this,but the time required for such schedules may become impractical.Further, continued additions of the organometallic additive will resultin more TiB₂ (4.52 g/cm³) relative to B₄C (2.52 g/cm³). For applicationssuch as personal armor this added weight would be unwelcome. This alsoasserts an upper limit to desirable titanium additions.

As can be readily understood by a skilled person, with the properselection of carbon content, pressureless sintering can take place at alower temperature. The carbon can hinder grain growth while the properselection of the titanium content can then attenuate the concentrationof carbon in the final sintered product. Thus, the combination of theproper selection of carbon and titanium can allow for sintering at a lowsoak temperature at the threshold of closed porosity (i.e. at theminimum relative density required for HIPing to be effective) whereingrain sizes are maintained small and carbon is consumed through reactionwith titanium, which will further increase the hardness of the material.That is, the carbon facilitates sintering, whose concentration is thenattenuated in the process of forming very hard, finely dispersed TiB₂phases. Also, by using a narrow particle size distribution B₄C powder(as a starting source of boron carbide) and the use of sintering soaktemperatures at the threshold of closed porosity, microstructures withaverage grain sizes approaching the original median particle size of thestarting boron carbide powder can be attained. The combination ofinterdependent factors has led to B₄C-based articles of higher hardnessthan previously reported.

In short, the data reported herein indicates that by coating boroncarbide particles with a titanium compound, a powder composition may beattained which, when pressureless sintered as set forth herein, canresult in an article having a high hardness value. Coating the particleswith a titanium compound and a carbon containing compound, and preparingan article based on a method according to the present invention canresult in an article having a microstructure with fine grains (with amedian grain size less than 100% larger than the median particle size ofthe boron carbide powder used for preparing the article) of B₄C andTiB₂, and a diminished concentration of graphite, which translates intoa hard article suitable for, for example, use in the construction ofarmor.

Comparative Study

To compare the effect of the water-soluble C₆H₁₀O₈Ti.2(NH₄) (titaniumcompound) coating to providing titanium in the form of particulate TiO₂additions, a separate series of experiments are described herein,undertaken by one of the named inventors and described in C. S. Wiley,Ph. D. Thesis, Synergistic Methods for the Production of High Strengthand Low Cost Boron Carbide, Georgia Institute of Technology, PublishedDecember 2011, which is referred to in the background section. In theseexperiments, HD15 boron carbide powder (see Table 1 above forcharacteristics), TiO₂ powders (0.9 μm rutile TiO₂ or 32 nm anataseTiO₂) and organic binders were added to deionized water and homogenizedto form a uniform slurry. Thus, in these studies, the slurry was notprepared with a water soluble titanium-containing organometalliccompound. Rather, titanium oxide particles were used to prepare a slurrywith boron carbide particles.

The slurries so prepared were then spray dried using an ultra-sonicatingspray drier nozzle and a pilot plant scale spray dryer to obtain apowder composition containing a titanium source. Samples were thenprepared from the powder composition in the same manner as thatdisclosed in FIG. 1A and related description. The amount of titaniumused for the preparation of slurry samples were 0.5, 1, 3.0 and 5.0 wt%. Carbon additions to the various titanium-containing slurry sample was0 wt %, 1.2 wt % and 3.7%.

FIGS. 19A-19C graphically disclose, as a function of pressurelesssintering soaking temperature, the effect of using titania powder (0.9μm d₅₀) in the slurries. FIGS. 20A-20C graphically disclose the resultsof HIPing on the same samples after pressureless sintering.

Referring to FIG. 21, highest post-HIPed relative densities wereachieved using 2240° C. and 2260° C. pressureless sintering soaktemperatures (˜60° C. higher than high-hardness samples containing theliquid Ti additive), with a powder composition that contained titanium 1wt % and 3.7 wt % carbon. As shown in FIG. 21, hardness was generallyhighest for this pressureless soak temperature and carbon content.Hardness increased only slightly with increasing Ti content but only fora soak temperature of 2260° C. The highest measured hardness, with 5 wt% Ti addition, was 2884.5 kg/mm². The phases in this post-HIPed specimenwere the same as observed for B₄C HD specimens disclosed herein of highhardness: B₄C, graphite, and TiB₂ (FIG. 22). The microstructure of thissample is shown in FIG. 23A. As compared to the microstructure of thespecimen using the organometallic additive with the highest hardness(FIG. 15B), this microstructure shows larger grains of B₄C, C, and TiB₂,as well as a more heterogeneous distribution of second phases (C andTiB₂). Note that the grain sizes are in the order of 10 um, which ismuch larger than the grain sizes in some of the samples preparedaccording to the present invention.

FIGS. 24A-24C and FIGS. 25A-25C show the sintered and post-HIPedrelative densities for a sample prepared in same manner as thiscomparative study except with 32 nm TiO₂ additions to the slurry.Referring to FIG. 26, a sintering soak temperature of 2260° C. for asample prepared from a powder composition containing more than 1 wt % oftitanium and 3.7 wt % carbon additions facilitated the highestpost-HIPed relative densities. The hardness achieved after post-HlPing asample prepared from a powder containing 5 wt % Ti, 3.7 wt % C, and apressureless sintered at a soak temperature of 2260° C. was 2846.0kg/mm² (FIG. 26). The lack of improvement in highest achievable hardnesswith nano-scale particle addition of TiO₂ is likely attributable to thehigh agglomeration tendency of such fine particles. As shown in FIG.23B, the TiB₂ grains in the microstructure were in fact coarsened ascompared to the case in which TiO₂ was added as 0.9 μm d₅₀ particles.The best hardness values for the samples prepared based powdercompositions containing the two TiO₂ particle sizes were well below thehardness achieved with the C₆H₁₀O₈Ti.2(NH₄) additive (3137.6 kg/mm²)used in the examples disclosed above. Thus, the comparative examplesindicate that higher hardness values may be achieved when using atitanium organometallic as a titanium source in the slurry is used forthe preparation of the powder composition used for the green bodies.

What is claimed is:
 1. A solid article formed from a boron carbidepowder that includes boron carbide particles having a median particlesize, the solid article comprising: a sintered body that includes aboron carbide phase and a titanium diboride phase, the phases beingpresent in granular form, wherein the relative density of the sinteredbody is more than 99% and the hardness of the sintered body is more than2600 Vickers, and the median grain sizes of the grains of each phase isno more than 100% larger than the median particle size of the boroncarbide particles in the starting powder.
 2. The solid article of claim1, wherein said sintered body further includes a carbon-based phase,wherein said carbon-based phase is 0-0.5% of the volume of said sinteredbody.
 3. The solid article of claim 1, wherein said sintered bodyincludes 0.081-5 wt % titanium in the form of TiB2.
 4. The solid articleof claim 1, wherein up to 50% of grains of said phases are less 1 μm indiameter.
 5. The solid article of claim 1, wherein up to 40% of grainsof said phases are less than 0.5 μm in diameter.